1. Field of the Invention
The present invention relates to a projection-type display apparatus which separates a white beam from a light source into beams of three colors, red, blue and green, modulates these beams through light valves according to image information, and re-synthesizes and projects the modulated beams under magnification onto a screen through a projection lens.
2. Description of Related Art
A projection-type display apparatus is comprised of a light source lamp, a color separating means for separating a white beam from the light source lamp into beams of three colors, three light valves for modulating the separated color beams, a color synthesizing means for synthesizing the modulated beams again, and a projection lens for magnifying and displaying a light image obtained by the synthesis onto a screen. As light valves, liquid crystal panels are generally used.
A conventional projection-type display apparatus having such structure in which a uniform illumination optical device referred to as an optical integrator is incorporated in a light source thereof is well known. For example, U.S. Pat. No. 5,098,184 discloses a projection-type display apparatus having such optical integrator incorporated therein. This patent publication also discloses a color synthesizing means consisting of dichroic mirrors arranged in the shape of an X. An ordinary color synthesizing means is constituted by dichroic mirrors each of which has a dielectric multilayer film on a glass plate.
Such projection-type display apparatus provided with a mirror composite system in which a color synthesizing means is constituted by dichroic mirrors has the following disadvantage. Each dichroic mirror is an optical element which is rotationally asymmetrical about the center axis of a projection lens. Therefore, astigmatism arises in an image on the screen, and a Modulation Transfer Function (MTF) representing the transfer characteristic of a projection optical system is lowered. As a result, the image is blurred and sharpness thereof is reduced. In a case in which the size of a liquid crystal panel is large relative to the number of pixels, in other words, when the pixel pitch is large, the lowering of the MTF does not cause such a large problem. However, when the pixel pitch is small, for example, as in a liquid crystal panel using a polysilicon TFT as a switching device, such lowering cannot be ignored.
Furthermore, a conventional projection-type display apparatus having a prism composite system in which a color synthesizing means consists of a dichroic prism is well known. The dichroic prism is an optical element which is rotationally symmetrical about the center axis of a projection lens. Therefore, astigmatism caused by this prism can be easily removed by the design of the projection lens. The MTF of the projection-type display apparatus having such a prism composite system is generally superior to that of the above-mentioned display apparatus having the mirror composite system. Accordingly, such apparatus is suitable in the case in which a liquid crystal panel having a small pixel pitch is used as a light valve.
Another type of conventional projection-type display apparatus is disclosed in, for example, U.S. Pat. No. 4,943,154. In this apparatus, the decrease in the amount of light and the unevenness in color are restricted by equalizing the optical path lengths (the distances between a light source and liquid crystal panels) of beams of three colors in a color separating means. In other words, a light transmitting means constituted by a relay lens, a field lens and so on is interposed on an optical path of the beam having the longest optical path length in the apparatus disclosed in the specification of this patent, thereby optically equalizing the optical path lengths of the color beams.
However, in this apparatus, while the light amount of the color beam having the longest optical path length is not reduced, the brightness distribution thereof is made to do a complete about-face by the relay lens. Therefore, if the initial brightness distribution is not axially symmetrical, color unevenness arises in the display on a screen, and the quality of the display is degraded. Although such color unevenness does not arise if the brightness distribution of the beam is axially symmetrical, in fact, the brightness distribution is normally made axially asymmetrical by the displacement of an attachment position of a light source lamp and the slight asymmetrical properties of the light source lamp and a reflecting mirror.
In a projection-type display apparatus, it is desirable to increase the luminous intensity of an image to be projected, and to obtain an image quality close to that of the image directly viewed on a CRT, without unevenness in color and luminous intensity. For such purpose, it is preferable to use a prism composite system having a good transfer characteristic as a color synthesizing system. It is also preferable to efficiently illuminate a liquid crystal panel with uniform brightness by using an optical integrator in a light source portion. However, if the optical integrator is used in a case in which the optical path lengths of the beams in the color separating system are different, the decrease in the amount of light and change of brightness distribution of the beam having the longest optical path are remarkable. This results in color unevenness and a change in color temperature of a projected image. Therefore, a sufficient effect of the optical integrator cannot be shown. Furthermore, when the optical integrator is used in the light source portion, the conventional art cannot be utilized as it is. In other words, since a diverged beam from a plane light source which exists in a finite position (a beam outgoing plane of the optical integrator) away from the liquid crystal panel illuminates the liquid crystal panel, the illumination with the optical integrator is basically different from the illumination from a point light source existing at an infinite distance from the liquid crystal panel, as in the arrangement of the conventional art.